Superconductivity is a condition existing within certain materials which, when cooled below a critical temperature T.sub.c, have essentially zero resistance to the flow of current. This condition is maintained as long as the current does not exceed the critical current density J.sub.c and the magnetic field does not exceed the critical field H.sub.c2. It is desirable to increase these critical limits in order to create more powerful electromagnets.
So far, critical current densities have been achieved of up to about 3500 A/mm.sup.2 at about 5 Tesla. It is believed that J.sub.c 's and H.sub.c2 's can be enhanced considerably by use of our invention. In one example, a theoretical critical current density was predicted of about 20,000 A/mm.sup.2 @5 Telsa if pinning centers are introduced by controlled and predetermined spacing having a laminar structure which continuously matches the flux line lattice.
Improvements in the critical current density have been achieved in the past through reduction in filament diameter, by selection of metals and alloys, and by use of pinning centers (defects). The latter, however, has not been done on a uniform and controlled basis.
Improvements in current density through conventional processing techniques have been accomplished, especially with niobium titanium, by random introduction of dislocations, grain boundaries, and use of alpha-titanium precipitates as pinning defects. The results have achieved current densities less than are theoretically predicted (6000 Amm.sup.-2) if a linear summation model is assumed. See C. Meingest, NbTi Workshop, Madison, Wisc., Nov. 1986.
Our invention provides controlled, uniform, pinning centers with predetermined spacing which match the flux line lattice (FLL) continuously. This is accomplished by designing the superconductor filaments to form continuous pinning centers in layers (laminar pinning centers) with spacing being predetermined and related to the fluxoid lattices. The use of drawing and extrusion techniques combined with this design yields a practical superconductor.
Examples of drawing and extrusion techniques are given in Garwin et al. U.S. Pat. No. 3,370,347 and Braginski et al. U.S. Pat. No. 4,411,959. Examples of making flux traps (pinning centers) are found in Martin Et al. U.S. Pat. Nos. 3,429,032 and 3,676,577. Discussions of flux pinning will be found in Raffy and Renard, "Critical Currents and Pinning Effect in Superconducting Alloy Films Spatially Modulated in Concentration", Solid State Communications, Vol. 11, p. 1679 (1972); and Yetter, Kramer, and Ast, "Flux Pinning by Thin Chromium Layers", Jour. of Low Temperature Physics, Vol. 49, p. 227 (1982). However, the literature does not appear to suggest the creation of uniformly continuous pinning centers of predetermined spacing by mechanical metallurgy.